Great tank! I like the look of the 50L over a standard 55, too. I didn't even know a 50L existed! I really want a poly too, but am worried it would take out medium sized fish (5"-6") once it became an adult. If it would be safe around medium sized fish, you could consider a school of Glossolepis incisus. The males are very pretty! I've seen people keep only 2-3 males with no females and not experience problems with the males fighting.